I think some (many) people who carry a gun don't sufficiently prepare their mindset for having to use it. I've seen the toughest guy I knew turn tail and run when he faced having to take an action that could have cost him his life if it went sideways (not gun or LE related).
Lots of folks out there carrying guns that have not made the mental decisions and defined their own "red lines", to shoot if necessary. That kind of uncertainty is what I believe drives folks to overreact to non-threats. They hung themselves out on the branch too far and they're in fear of having to take an action for which they are not sufficiently trained (MUC) or mentally prepared.
Disclaimer: I'm speaking from observation and self evaluation, not from any substantial experience.